Thyroid hormone active drugs are known for both therapeutic and prophylactic treatment of thyroid disorders. For example, levothyroxine sodium is prescribed for thyroid hormone replacement therapy in cases of reduced or absent thyroid function in ailments such as myxedema, cretinism, and obesity. See, for example, Post and Warren in Analytical Profiles of Drug Substances, Vol. 5, Florey (ed.); Academic Press, New York (1976), pp. 226-281. Levothyroxine sodium is quite unstable, hygroscopic and degrades rapidly when subjected to high humidity, light or high temperature. See, for example, Won, Pharm. Res.9(1):131-137, 1992. Because of the physico-chemico properties of the drug, many levothyroxine sodium formulations have short stability duration, worsened under conditions of high humidity and temperature. Tablets may decompose approximately 1 percent per month. Gupta et. al., J. Clin. Pharm. Ther. 15:331-335, 1990. The stability problem has been so widespread that some drug companies marketing levothyroxine sodium tablets have been forced to recall various batches due to lack of stability.
Formulations containing levothyroxine sodium have been known since the late 1950s. There have been attempts to develop more stable dosage formulations of levothyroxine sodium. For example, U.S. Pat. No. 5,635,209 discloses levothyroxine sodium in combination with potassium iodide as part of a stabilizing excipient. In the manufacture of this formulation, levothyroxine sodium was first mixed with microcrystalline cellulose, and then added to a dried granulation of potassium iodide and microcrystalline cellulose. The formulation purportedly provided increased active drug potency over a three month period in comparison to then commercially available formulations.
U.S. Pat. No. 5,225,204 teaches a complex of levothyroxine sodium and a cellulose, polyvinylpyrrolidone or poloxamer. The formulation may be prepared by dissolving the drug complex in a polar organic solvent, adding a cellulose carrier to the liquid, and drying the resulting mixture to obtain a complex of levothyroxine sodium and polyvinylpyrrolidone or poloxamer adsorbed on the cellulose carrier. Tests of such combinations yielded stability results at best equal to commercially available preparations such as those described in U.S. Pat. No. 5,955,105, and in some cases substantially worse. The inventors of this stabilized composition teach one of skill in the art away from the use of carbohydrates in levothyroxine sodium formulations, stating that instability of the dosage form was the result of an interaction between the active drug substance and carbohydrate excipients.
U.S. Pat. No. 5,955,105 teaches that levothyroxine is relatively stable in pure form and that the instability of levothyroxine is due to its interaction with particular excipients. The patent teaches that thyroid hormones, particularly levothyroxine sodium, are compatible with carbohydrates, such as starch and maltodextrin, but incompatible with lactose, glucose and sucrose. The patent teaches a formula for direct compression levothyroxine sodium dosage forms that contains a soluble polysaccharide, designed to eliminate the interaction between the drug and other excipients, and carbohydrate having a molecular weight greater than 500.
U.S. Pat. Nos. 7,195,779 and 7,052,717 teach storage-stable pharmaceutical compositions of thyroid hormones, such as levothyroxine sodium, are achieved by blending the active ingredient with stabilizing amounts of mannitol and sucrose, or mannitol, sucrose, and antioxidant butylated hydroxyanisole (BHA), to form a granulation intermediate. The patents teach that a stabilizing effect is achieved for levothyroxine sodium in its formulations due to the presence of the mannitol and sucrose, or mannitol, sucrose, and BHA, both at an early stage of manufacture and in the final dosage form.